Quantum wells are formed in semiconductor devices such as diode lasers, High Electron Mobility Transistors (HEMTs) used in low-noise electronics and infrared photodetectors used for infrared imaging. Particularly, a quantum well is a potential well that confines particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable at the de Broglie wavelength of the carriers (generally electrons and holes); leading to energy levels called “energy subbands”, i.e., the carriers can only have discrete energy values.
Quantum wells are formed in semiconductors by having a material, like gallium arsenide sandwiched between two layers of a material with a wider bandgap, like aluminum arsenide. These structures can be grown by molecular beam epitaxy or chemical vapor deposition with control of the layer thickness down to monolayers.
In order to achieve high mobility quantum well device structures, a key element is the ability to confine dopants in close proximity h intrinsic quantum well. Such a requirement is not easily met in many cases due to the uncontrolled diffusivity of such dopants. The dopants in a delta doped layer can diffuse or “spill into” the quantum well during the subsequent growth and annealing steps and hence degrade the device mobility/performance.
A partial solution to the problem of dopant out-diffusion from the delta doped layer during subsequent dopant activation annealing steps is the use of ultra fast ramping RTA (rapid thermal annealing). This does not address dopant diffusion/spread entirely though since dopants can also diffuse during the remainder of the growth process for the surrounding high energy gap material. Furthermore many other subsequent processes such as metallization, spacer formation, etc. may not be compatible with the ultra low thermal budget requirements for maintaining the delta doped layer.